1. Field of the Invention
The present invention relates to a method and apparatus for applying a liquid coating composition (hereinafter referred to merely as a "coating solution") on a long, continuous and flexible support (hereinafter referred to merely as a "web") which is conveyed past a source of the coating solution.
2. Description of the Prior Art
A so-called, multi-layer slide bead coating apparatus, disclosed by Russell et al in U.S. Pat. No. 2,761,791, has been widely used for applying a coating solution onto a surface of a web. In accordance with this apparatus, the amount of coating solution running down a plurality of tilting surfaces (slide surfaces) is controlled so that it forms a bead at a position where the coating solution comes into close contact with the web being conveyed, and coating is conducted through the bead. Therefore, in apparatuses of this type, it is necessary that the bead be kept stable so that satisfactory coating characteristics can be obtained. However, as the coating speed is increased, it tends to become difficult to maintain such a stabilized bead condition. In addition, it also becomes difficult to maintain a stabilized bead condition as the viscosity and/or the surface tension of the coating solution is reduced.
Japanese patent application (OPI) No. 133067/81 (the term "OPI", as used herein, means a "published unexamined Japanese patent application") discloses a coating device which is capable of discharging at least one coating solution 6, 7 from a coating solution outlet onto a surface of a die block 3, as shown in FIG. 1. The discharged coating solution is allowed to flow over the die surface and, thereafter, is used to be applied to a surface of a web 1 which is advanced around backing roll 2 along a direction A' which is substantially perpendicular to the coating solution. The roll rotates along the direction A, and second and third die blocks 4 and 5 are provided for channeling the first and second coating solutions onto the first die block 3. The first coating solution 6 and the second coating solution 7 are applied to the web 1 to form a first coating layer 9 and a second coating layer 10, respectively. The first coating solution 6 and the second coating solution 7 move as a laminar flow on the plane of the die block 3 and reach the lip of the die block 3 by the action of surface tension, thereby forming a bead 8 between the die block 3 and the web 1. As the web 1 advances, the coating solutions 6 and 7, in superimposed layers, are applied to the web 1 while being stretched to more than twenty or thirty times their original length to form coating films 9 and 10.
In the conventional coating apparatus shown in FIG. 1, the coating solution depends only on surface tension forces for its movement on the plane of the die block 3. Therefore, the thickness of the coating solution which flows over the surface of the die block 3 must be increased to exceed a predetermined value before a bead is formed. Furthermore, no excessive force is exerted on the bead while coating the web 1. It is thus possible to apply a coating solution which has a viscosity of 10 cp or less at high speeds and in a thin layer form. However, when applying a coating solution which has a low viscosity and good wetting properties (i.e., a viscosity of 2 cp or less and a surface tension of 25 dyne/cm or less, such as an organic, solvent-based coating solution), an undesirable phenomenon occurs in which the bead drops from the lip of the coating apparatus in a direction which is opposite to the arrow A'.
U.S. Pat. No. 2,681,294 discloses a bead-stabilizing technique in which, as illustrated in FIG. 2, a coating solution in a coating hopper 11 is fed through a slot 12 to form a coating film 9 on the surface of a web 1. The lower side of a bead 8 is maintained in a pressure-reduced condition by providing a vacuum chamber 13 and operating a valve 14, a liquid trap 15, a manometer 16, and a vacuum pump (not shown) to produce a difference in pressure between the upper side and the lower side of the bead 8. In accordance with this method, when a coating solution which has a low viscosity and good wetting properties (such as an organic, solvent-based coating solution) is coated onto a support, the bead-dropping phenomenon referred to above cannot be avoided, and the bead 8 is broken.
FIG. 3 shows a coating apparatus as described in Japanese patent application (OPI) No. 47039/77. A backing roll 2 is rotated in a direction indicated by the arrow B to allow a web 1 to advance in the direction indicated by the arrow B'. A coating solution is introduced into a cavity 17 in a coating hopper 16 by means of a pump 18 in an amount necessary to form a desired layer thickness on the web 1. The coating solution flows through a slot 19 and is allowed to slide down an inclined surface 20 to form a wedge-shaped coating solution reservoir 8 in a clearance between the coating hopper 16 and the web 1. Then, compressed air at a pressure of about 50-500 mm H.sub.2 O is applied to the lower end of the coating solution reservoir from a nozzle 22 of an air compression chamber 21 to form a meniscus. The nozzle 22 and the air compression chamber 21 are located below and adjacent to the coating hopper 16. In this way, a uniform, thin layer is coated at a rate between 50 and 100 m per minute. Since this coating method is based on the same principle as that disclosed in the above-described U.S. Pat. No. 2,681,294 (FIG. 2), when a coating solution which has a low viscosity and good wetting properties (such as an organic, solvent-based coating solution) is applied to the web, a part of the bead is broken. The presence of the air compression chamber 21 thus has an adverse effect on the stability of the bead.
Accordingly, the conventional methods and apparatuses are unsuitable for applying a coating solution which has a low viscosity and good wetting properties (e.g., an organic, solvent-based coating solution) onto the surface of a support since the solution bead tends to be unstable.
It is noted that the term "low viscosity", as used herein, generally means a viscosity of 10 centipoises (cps) or less, and the term "low viscosity and good wetting properties", as used herein, means that the viscosity is 2 cps or less, and the surface tension is 25 dynes per centimeter (dyne/cm) or less. Furthermore, the term "organic solvent" is used herein to mean a solvent-based coating solution in which acetone, methanol, ethanol, methyl chloride, butanol, methyl glycol, methyl ethyl ketone, and ethyl cellulose, for example, are used alone, in combination with each other, or in combination with water, as usually used in the preparation of photographic materials and recording materials.